The invention relates to a method for producing a high temperature superconductor (=HTS) coated conductor, wherein a buffer layer and an HTS layer are deposited on a substrate.
Such a method is known from U.S. Pat. No. 7,491,678 B2.
Superconductors are used to transport electrical current at low resistance, and thus at low power losses. Superconductors require cryogenic cooling in order to assure the superconducting state. High temperature superconductor (=HTS) materials have a relatively high critical temperature Tc, as compared to conventional superconductors, and therefore are less expensive in cooling. In particular, some HTS materials allow the use of liquid nitrogen for cooling. On the other hand, the HTS materials exhibit rather high critical current densities at low temperatures of 3 to 20K. This performance is important for high field magnets used in e.g. NMR where fields are so high that the current performance of low temperature superconductors (typically used in this area) becomes insufficient.
Known HTS materials are ceramic, and may be (and typically are) prepared as thin films on a substrate for practical applications (“coated conductor type”). Generally, in order to achieve a high current carrying capacity, the thin HTS films should be highly textured, practically single crystalline, with a defect density as small as possible. It is known to use buffer layers between a substrate and a HTS thin film in order to provide the crystalline structure of the thin HTS film, or to improve the crystal quality of the thin HTS film.
However, in the regime of use of HTS materials, the HTS materials act as type II superconductors, meaning that magnetic flux may penetrate into a HTS layer, typically as isolated “flux pipes” (in German “Flussschläuche”). The density and position of the flux pipes depends on the magnetic field present (which may be an external field and/or a field originating from the electrical current running through the HTS layer, “self field”). During use of the HTS coated conductor, the magnetic field typically varies, and the flux pipes redistribute accordingly. This redistribution or movement of the flux pipes causes electrical losses, and may decrease the critical current and the critical magnetic field of the HTS coated conductor at which the HTS material quenches (i.e. becomes normally conducting).
It is known that “pinning centers” may reduce losses and increase the critical current and the critical magnetic field in a type II superconductor. The pinning centers make the flux pipes stay at the pinning centers when the magnetic field varies.
U.S. Pat. No. 7,491,678 B2 proposes to deposit insularly nano dots, typically made of Ag, Mg or Zn, on a flat, buffered substrate surface, and then to deposit a HTS film on top of the buffered substrate surface provided with the nano dots. The HTS film grows with columnar defects above the nano dots, and the columnar defects act as pinning centers for the magnetic flux. However, the nano dots may easily introduce more defects into the HTS film than desired, what decreases the critical current and the critical magnetic field again.
U.S. Pat. No. 7,642,222 B1 describes a cubic oxide material substrate equipped with a buffer layer of rocksalt type including particulate outgrowths, resulting in a rough surface morphology of the buffer layer. The HTS film is grown of top, wherein the particulate outgrowths serve as pinning centers. In this method, the surface morphology of the buffer layer is difficult to control, and extended defects are introduced into the HTS film, what limits the positive effect of the pinning centers again.
It is the object of present invention to provide a simple method for producing a HTS coated conductor with reduced losses, and with improved critical current and critical magnetic field.